Described herein is a process for coating a substrate with a polymer by immersing a heated substrate in a fluidized bed of polymer particles. After removal of the coated substrate from the fluidized bed, additional heat can be applied to level the coating and, if the polymer is thermosetting, to effect cure.
The coating of substrates, such as metals, is useful for aesthetic purposes and for practical purposes such as corrosion protection. Many types of coating materials and processes for utilizing these coating materials are known in the art. For environmental reasons, there is a trend to using coating materials that emit low levels of organic volatiles, and preferably no volatiles at all, during the coating process.
One method which creates low levels of volatiles in the coating process is powder coating applied by fluidized bed. One drawback to the process as it is currently practiced is that relatively thick coatings are produced because of the lack of appreciation of how to control coating thickness to consistently obtain thinner coatings. In order to overcome this shortcoming, electrostatic spraying is sometimes used. However, the electrostatic process requires elaborate equipment, and does not typically coat all surfaces within an object.
Descriptions of typical powder coating methods are found in Jilek, "Powder Coatings", Federation of Societies for Coating Technology, Blue Bell, Pa., U.S.A., October 1991, pages 7 to 35; Landrock in Encyclopedia of Polymer Science and Technology, Vol. 3, McGraw Hill Book Co., New York, 1965, pages 808 to 830; Landrock in Chem. Eng. Progress, Vol. 63, No. 2, pages 67 to 73; Richart, Plastics Design and Processing, July 1962, pages 26 to 34; and Kroschwitz, Ed., Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 6., John Wiley & Sons, New York, 1993, pages 635 to 661. Fluidized beds are well-known in the art, see for instance, Elvers, et al, Ed., Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. B4, VCH Verlagsgesellschaft mbH, Weinheim, 1992, pages 240 to 274. With respect to making spherical particles of copolymer, see U.S. Pat. No. 3,933,954 and U.S. Pat. No. 4,056,653.
None of these references describes a fluidized bed process into which is dipped a substrate, heated just to the temperature at which it causes tackiness of the polymer particles that contact the substrate, or modestly higher, together with control of the particle size. By heating the substrate significantly above the melting point of the polymer, the art regularly achieves coating thicknesses exceeding what is useful in certain practical applications. For instance, typical procedures taught in the art produce coatings too thick for automotive applications, as well as other applications where thicknesses of 150 micrometers, even significantly below 150 micrometers, are desired. This deficiency has been a primary factor in slowing the growth of powder coating applications.